Do shallow clones of submodules in x.py

[bjorn3]

https://git-scm.com/docs/git-submodule#git-submodule---depth

[est31]

I think that has been tried but was abandoned because switching back to some older revision didn't work well.

[est31]

cc #42208

[Mark-Simulacrum]

Yeah, this wouldn't really work since we don't know what depth we need to get the commit we want in each submodule. I don't think this is viable -- and we should probably just close it, though if anyone has ideas I want to hear them so I'll leave it open for now.

[Mark-Simulacrum]

Closing -- this doesn't work in practice.